Gene targeting provides the means for creating strains of mice with mutations in virtually any gene. This technology permits evaluation of the function of genes in an intact mouse and the systematic dissection of the most complex of biological processes, including the study of cancer, immunology and neurobiology. In this grant we are seeking to extend the gene targeting technology beyond its common use to generate "knockout" mice. Emphasis will be placed on improving conditional mutagenesis protocols by incorporating, for example, Cre-based and FIp-based self-excision cassettes to allow more rapid removal of unwanted DNA sequences when the cassette passes through the male germ line. We also propose to generate a Cre-loxP-based conditional mutagenesis system that can be modulated temporally. Such extensions should improve the gene targeting technology and make it more useful for the generation of mouse models of human genetic disease and the study of human medicine. [unreadable] [unreadable]